A knowledge of specific synaptic mechanisms is fundamental to understanding how the nervous system functions. These mechanisms are best understood at those synapses where there is confident knowledge of the specific transmitter involved. The overall objective of this study is the identification and characterization of elements of a simple nervous system in which to study serotonergic transmission. Setotonin is a candidate for the role of cardioinhibitory transmitter in the arthropod Limulus polyphemus. The general characteristics of the arthopod nervous system makes this system attractive for consideration as such a model. Thus the immediate objectives of the proposed study are: (1) to establish the cardioregulatory role of serotonin by biochemical and electrophysiological methods; and (2) to biochemically characterize individual, physiologically identified cardioinhibitory and cardioexcitatory cells. In these identified cells, sensitive and specific radiochemical assays in microbiochemical procedures will be used to measure single cell levels of transmitter system entities. The analysis of this data should make possible the identification of the biochemical substrata of neuronal transmission specificity.